Doppler effect

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun change in the apparent frequency of a wave as observer and source move toward or away from each other